Bicycle rim brake

ABSTRACT

Responsive to a hydraulic piston advancing out of a piston housing in a second brake arm and contacting a piston cam surface of a first brake arm, the second brake arm pivots around a second pivot. As this is happening, a cam surface in the second brake arm lifts a contact surface of a first force transfer member of the first brake arm, imparting torque to the first brake arm which then pivots around a first pivot. The cam surface is shaped to impart synchronous motion to the first and second pad holders. Splitting one of the brake arms with a centering member permits a centering adjust feature.

BACKGROUND

The present disclosure is directed to bicycle brakes and, moreparticularly, to bicycle rim brakes.

In general, there are two types of rim brakes. One is referred to as asingle pivot rim brake and the other is referred to as a two-post rimbrake. The two-post rim brake usually includes a first brake armpivotable about a first axis and a second brake arm pivotable about asecond axis. The first and second brake arms include a link orconnection therebetween for moving the brake pads at the same rate. Adisadvantage of a two-post brake is the braked induced frame/fork postloads. There is a need to provide a two-post rim brake with reducedframe/fork post loads.

SUMMARY

According to one aspect, a rim brake is provided that uses only onehydraulic piston assembly to actuate opposed angular motion of first andsecond brake arms. The first brake and has a first pivot adapted forpivotal attachment to the bicycle. A first brake pad holder of the firstbrake arm downwardly depends from the first pivot. The first brake padholder is configured to move in a first arcuate direction around thefirst pivot. The second brake arm has a second pivot adapted for pivotalattachment to the bicycle. A second brake pad holder of the second brakearm downwardly depends from the second pivot. The second brake padholder is configured to move in a second arcuate direction around thesecond pivot opposite the first arcuate direction. The second pivot ishorizontally spaced from first pivot. The only one hydraulic pistonassembly includes a piston slidably mounted in a piston housing of thesecond brake arm, the piston advancing along a piston axis responsive tohydraulic fluid pressure to cause a non-horizontal force to be exertedon the first brake arm to move the second brake pad bolder in the secondarcuate direction around the second pivot.

According to another aspect, a rim brake has first and second brakearms. The first brake arm has a first pivot from which downwardlyextends a first pad holder. The second brake arm has a second pivot fromwhich downwardly extends a second pad holder. Responsive to actuation ofthe brake, the first pad holder pivots in a first angular direction, andthe second pad holder pivots in ail opposite, second angular direction.The second pivot is horizontally spaced from the first pivot. The secondbrake arm further includes a biasing member that upwardly extends fromthe second pivot. This biasing member pivots around the second pivotwith the second pad holder as a unit. The second brake arm further has aforce transfer member that upwardly extends from the second pivot. Thisforce transfer member is operable to transmit torque in the secondangular direction to the second pad holder upon actuation of the brake.A centering member, such a set screw, defines the angular position ofthe biasing member relative to the force transfer member. The centeringmember is adjustable to move the second pad holder without moving thefirst pad holder, thereby providing a center adjust function.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects and their advantages can be discerned in the followingdetailed description, in which like characters denote like parts and inwhich:

FIG. 1 is an elevational view of a bicycle employing rim brakesaccording to the one embodiment;

FIG. 2 is a perspective view of a rim brake according to one embodiment;

FIG. 3 is a front view of the brake shown in FIG. 2;

FIG. 4 is a sectional view taken substantially along line 4A-4A of FIG.3;

FIG. 5 is part-sectional view of the brake shown in FIG. 3, diagrammingcertain spatial relationships;

FIG. 6 is a back view of the brake shown in FIG. 2;

FIG. 7 is a transverse sectional view of the brake shown in FIG. 3, thesection taken substantially orthogonally to a center plane;

FIG. 8 is a detail of FIG. 7 showing structure of a piston cam surface;

FIG. 9 is a transverse sectional view of the brake shown in FIG. 3, thesectional plane being parallel to the sectional plane of FIG. 7;

FIGS. 10A and 10B are force diagrams of different components of thebrake shown in FIG. 2;

FIG. 11 is a right side view of the brake shown in FIG. 2 andrepresentative structure of a bicycle frame to which the brake ismounted;

FIG. 12 is a part-sectional front view of the brake shown in FIG. 3, ina non-actuated condition;

FIG. 13 is a part-sectional front view of the brake shown in FIG. 3, butshown in an actuated condition;

FIG. 14 is a part-sectional view of the brake shown in FIG. 3, but shownin an open condition;

FIGS. 15 and 16 are perspective and front views of an alternativeembodiment in which a centering adjust is on a different brake arm;

FIG. 17 is a perspective view of an alternative embodiment having nocentering adjust;

FIG. 18 is a perspective view of an alternative embodiment in which oneend of a coil return spring is attached to a brake mount;

FIG. 19 is a perspective view of an alternative embodiment in which thebias member is a torsion spring;

FIG. 20 is a perspective view of an alternative embodiment including atorsion return spring wound around a brake mount; and

FIG. 21 is a front view of a cable pull embodiment.

DETAILED DESCRIPTION

FIG. 1 is an elevational view of a bicycle indicated generally at 100,which may be used to implement rim brakes disclosed herein. While theillustrated bicycle is a road bike, the brakes may be employed on anybicycle using one or more rim brakes, including touring bicycles,commuter bicycles, mountain bikes, cyclocross bikes and triathlon bikes.Bicycle 100 has a frame 102 that includes a head tube 104 and left andright seat stays (right seat stay 106 being shown). A fork 108 rotateswithin the head tube 104. The fork 108 rotatably mounts a front wheel110 to the rest of the bicycle 100. Front wheel 110 has an axis 112 anda wheel rim 114. The wheel rim 114 has a rim sidewall or other annularbraking surface 116 against which a brake pad (described below) may beemployed. The annular braking surface 116 is disposed at a predeterminedradius from the wheel axis 112. A front rim brake 118 may behand-actuated by a user to apply fractional force to opposed annularbraking surfaces 116. The front rim brake 118 is mounted to the fork 108on left and right axial mounts in a manner which will be describedbelow.

In the illustrated embodiment, a rear wheel 120 is rotatably mounted tothe rest of the bicycle 100 at the junction of its seat stays and chainstays, right chain stay 121 being shown. The rear wheel 120 has a rearwheel axis 122 and a rim 124. Sidewall or other opposed annular brakingsurfaces 126 of rim 124 are a predetermined radius away from the rearwheel axis 122. The radii of wheels 120 and 110 typically are the samebut don't have to be. Further, while a diamond frame 102 that includesboth chain stays and seat stays is illustrated, some frame designs omitthe seat stay or replace it with more elaborate shock-absorbingapparatus, and brake(s) disclosed herein may be used in conjunction withthese other frame types.

In the illustrated embodiment, a rear rim brake 128 is mounted on dualposts (not shown in the FIG. 1) that are in turn formed on the left andright seat stays of the frame 102. In other embodiments, the rear rimbrake 128 may be mounted elsewhere on the frame 102 to be adjacent therear annular braking surfaces 126, such as on a seat tube 130 of frame102. The rim brake 200 described below may be employed as a front, rimbrake 118, a rear rim brake 128 or both. Unless it is being steered leftor right, in use bicycle 100 occupies, and moves in, a vertical centerplane P (FIG. 5).

A handlebar 132 rotates as a unit with fork 108. In one conventionalarrangement, mounted on the handlebar are two hand-operated brake levers(rear brake lever 134, as typically mounted on the right side ofhandlebar 132, being shown) that are operated by the user to actuate thebrakes 118, 128. In the embodiment illustrated in FIGS. 2-14, the brakes118, 128 are hydraulic brakes, each of which incorporate a piston withina slave cylinder (described below) that moves responsive to the pressureof hydraulic fluid. As applied to hydraulic embodiments, brake lever 134has a master piston and cylinder (not shown) incorporated within it andis connected by a hydraulic brake line 136 to rear brake 128. Similarly,the left brake lever has a master piston and cylinder located within it,and is connected by a hydraulic brake line 138 to the front brake 118.An increase in the fluid pressure of line 136 will cause the piston(described below) in the slave cylinder in brake 128 to move along thecylinder axis, causing brake 128 to clamp against annular brakingsurface 126. An increase in the fluid pressure of line 138 will causethe piston in the slave cylinder in brake 118 to move along the cylinderaxis, causing brake 118 to clamp against annular braking surfaces 116.

In another embodiment, the hydraulic brake lines or hoses 136, 138 arereplaced by Bowden cables, which exert a rider-varied degree of tensileforce on cable-actuated rim brakes to which they are connected. Acable-actuated embodiment will be described below.

FIGS. 2-14 illustrate one hydraulic brake according to one embodiment. Abrake indicated generally at 200 has a first brake arm 202 and a secondbrake arm 204. In the view shown in FIG. 3, and as mounted on mounts ofa pair of seat stays (not shown), the first brake arm 202 is the leftbrake arm and the second brake arm 204 is the right brake arm, althoughthe entire structure can be easily reversed in mirror image.

First brake arm 202 is pivotably attached to a first brake mount (notshown) on a first pivot or first axis 206 by suitable fasteners 207.Second brake arm 204 is pivotably attached to a second brake mount (notshown) by suitable fasteners 209 on a second pivot or second axis 208.Fasteners 207, 209 can each include a shaft and bushings. The first axis206 is parallel to the center plane P and is a predetermined distancefrom plane P in a left outboard direction (FIG. 5). The first axis 206further is tangential to a vertical radius drawn from the bicycle wheelaxis (112 or 122) with which it has been associated. The second axis 208is likewise parallel to the center plane P and is spaced from it in aright outboard direction that that is equal to the spacing of first axis206 from plane P (FIG. 5). The first axis 206 is horizontally displacedfrom the second axis 208 such that a horizontal reference Line 205extends therebetween (FIG. 5). The axis 208 further is tangential to avertical radius drawn from the bicycle wheel axis (112 or 122) withwhich it has been associated.

First brake arm 202 has a “downwardly” depending first brake pad holder210 having an end 212 that is displaced from first axis 206 toward wheelaxis 112 or 122. It should be understood that as used herein and asdescribing the structure of the different brake embodiments describedherein, the words “downward”, “downwardly”, “lower”, “upward”,“upwardly” and “upper” all refer to a radial distance towards or awayfrom wheel axis 112 or 122, depending on where the brake is installedand which wheel 110, 120 the brake is meant to brake; as installed, thebrake first and second axes 206, 208 will often be at a considerableangle to the horizontal. A brake pad 214 is affixed to the end 212 so asto reside within an arc (around first axis 206) that intersects anannular braking surface 126A of rear wheel 120 (while the use of brake200 as braking the rear wheel 120 is shown in particular in FIGS. 2-13,the description also applies to brake 200 being used as a front brake tobrake wheel 110). Likewise, second brake arm 204 has a downwardlydepending brake pad holder 215 with an end 216 that is displaced fromaxis 208 toward wheel axis 122. A brake pad 218 is affixed to the end216 so as to reside within an arc (around axis 208) that intersects theopposed annular braking surface 126B of wheel 120.

The brake 200 includes a force transfer mechanism 201 between the firstand second brake arms 202, 204 configured to move the first pad holder210 in the first arcuate direction about the first pivot 206 as thepiston advances. The force transfer mechanism 201 includes first andsecond force transfer members 220, 228. The first brake arm 202 has thefirst force transfer member 220 which, in this embodiment, extends in aninboard and upward direction to an end 222. In this illustratedembodiment, a contact surface such as a roller 224 is affixed to thetransfer member end 222 along an axis which is parallel to first axis206. The first brake and 202 further has a piston cam surface 226 thatis disposed in an inboard and upward direction from the first axis 206.The piston cam surface 226 is convexly arcuate relative to first axis206. The piston cam surface 226 may be disposed forwardly from thetransfer member end 222; in art alternative embodiment the transfermember end 222 may be disposed forwardly from the piston cam surface226. In the illustrated embodiment, the transfer member 220 and thepiston earn surface 226 are integrally formed as by machining and/ormolding and adjoin each other. Transfer member 220, first brake padholder 210 and piston cam surface 226 freely rotate around the firstaxis 206 as a unit.

The second brake arm 204 has the second force transfer member 228 thatextends upwardly and in an inboard direction from the second axis 208.The second force transfer member 228 includes a piston housing orcylinder 230 formed along a piston axis 232. In the illustratedembodiment the piston axis 232 rotates around second axis 208 as thebrake 200 is being actuated. Nonetheless, throughout its range ofmotion, the piston axis 232 remains inboard from the first axis 206. Thepiston housing 230 may be mostly or completely located on the other sideof center plane P relative to second axis 208.

As particularly seen in FIGS. 7 and 9, a slave piston 234 slides withina cylinder 235 defined by the piston housing 230 along piston axis 232.The piston 234 extends downwardly out of piston housing 230 responsiveto an increase in fluid pressure in hydraulic hose or line 136. An endof hose 136 is inserted into a chamber 231 and is fixed in place withthe aid of a nut 233 and a compression fitting 237. The fluid withinhose 136 and chamber 231 is in fluid communication with hydraulic fluidwithin chamber 236 via passages 243 and 239. The position of a pistonstop 238 is set by a threaded barrel adjuster 240. The piston stop 238has an annular seal 242, and the piston 234 has an annular seal 244,both engaging cylindrical sidewall 235 and acting to contain thepressurized hydraulic fluid within hydraulic chamber 236. A lowersurface of the piston stop 238 defines the upper limit of slave piston234, as will obtain when the brake pad holders are in their openposition (FIG. 14). The cylinder 235 further is ported via passage 239to a hydraulic fluid bleed port 241 (FIG. 4).

In the illustrated embodiment, the piston stop 238 may be advanced downor retracted up the piston axis 232 by one of two adjust mechanisms of abarrel adjuster 240. A coarse adjust or “quick release” lever 246rotates as a unit with a quick release stop 248. Quick release stop 246has relatively coarse threads which engage with a threaded upper bore250 of the piston housing 230. An inner bore 252 of the quick releasestop 248 has relatively fine threads which threadedly engage with theshaft of a micro adjust rod 241. When a user rotates the quick releaselever 246, the quick release stop and the micro adjust rod 241 willrotate with it as a unit, quickly advancing or retracting the piston 234up or down the cylinder 235, and quickly opening or closing the brakepads, as might be desired for removal and reinstallation of the wheel120.

The brake pad gap relative to the wheel rim or annular braking surfaces126 is set by twisting a barrel adjust housing 254 relative to the brakequick release lever 246. Barrel adjust housing 254 turns as a unit withmicro adjust rod 241, and will advance or retract rod 241 relative tothe quick release stop 248. A helical spring 256 places tensile forcebetween the inner bore 252 of the quick release stop 248 and the barreladjust housing, reducing or preventing undesired relative movement ofthese components.

As best seen in FIG. 8, piston 234 terminates at its lower end with apiston contact surface 258. In the illustrated embodiment this contactsurface 258 is a flat disk. Piston contact surface 258 meets the pistoncam surface 226 at a line of contact 260 that stays in alignment withthe piston axis 232. The convexly arcuate shape of piston cam surface226 is preselected so that the piston axis 232 goes through the line ofcontact 260 throughout the range of stroke of the piston 234. In analternative embodiment (not shown), the contact surface 258 or thepiston cam surface 226 may be replaced with a roller.

The second, force transfer member 228 of the second brake arm 204further has, formed on or within it, a second contact surface, such as atransfer cam surface 262. In the illustrated embodiment the transfer camsurface 262 is located in an upper and inboard direction from the secondaxis 208, but is displaced somewhat downwardly from the piston housing230 so as to be intermediate the piston housing 230 and the second axis208. In one embodiment, the position and shape of the transfer camsurface 262 are selected such that the speed with which brake pad 214moves toward or away from rim 126A equals the speed with which brake pad218 moves toward or away from the opposed rim 126B; the brake pads 214,218, and the arms 202, 204 to which they are attached, are then said tomove synchronously. For synchronous movement, it has been discoveredthat the shape of transfer cam surface 262 can be nearly circularlycylindrical in the illustrated embodiment, the synchronizing transfercam surface 262 has a radius 264 from a center 266 that is locatedinboard from first and second axes 206 and 208. The center 266 islocated downwardly from the transfer cam surface 262. In an alternativeembodiment, the roller 224 may be replaced with a cam surface (notshown) that slides on transfer cam surface 262. In a still furtherembodiment (not shown), the first brake arm 202 may be fitted with a camsurface and the second brake arm with a roller, opposite the arrangementshown.

In this Illustrated embodiment, the second brake and 204 is split intotwo parts; the second brake arm force transfer member 228, and a biasingmember or portion 268 that includes brake pad holder 215 and a centeringadjust body 270 that extends upwardly from the second axis 208. Bothportions 228 and 268 are rotatably mounted to second axis 208. Athreaded bore 272 in the centering adjust body 270 is disposed upwardlyfrom the axis 208 and accepts a centering member such as a set screw274. An end 276 of the set screw 274 abuts a surface 278 of thetransferring member 228. As set screw 274 is advanced into bore 272, thecentering adjust body 270 angularly displaces relative to transferringmember surface 278. As a result, brake pad holder 215 will move in aninboard direction, but brake pad holder 210 will not. Set screw 274 canbe rotated in an opposite direction to open brake pad holder 215relative to brake pad holder 210. This provides an ability to center thebrake pads 214, 218 around the wheel rims or annular braking surfaces126A, B.

In this embodiment, the and 202 is biased against arm 204 by a helicalreturn spring 280. A first end 282 of the return spring is attached to apost 284 on the first brake arm 202. A second end 286 of the returnspring is attached to a post 288 on an end of the centering adjust body270. Spring 280 urges together posts 284 and 288. Therefore, on theother side of the axes 206, 208, the brake pad holders 210, 215 arebiased to an open or retracted condition. The coil spring 280 also actsto urge the end 276 of the set screw 274 against surface 278 of thetransferring member 228 of the second brake arm 204, causing the secondbrake arm components 228 and 268 to rotate as a unit around second axis208.

In operation and referring to FIG. 5, a rider squeezes a brake lever(such as rear brake lever 134) on the handlebar 132 (see FIG. 1),causing a master cylinder therein (not shown) to put pressure onhydraulic fluid within hydraulic brake line 136. This causes the slavepiston 234 to downwardly extend from the piston housing 230 on thesecond brake arm 204. The lower contact surface 258 of the piston 234stays abutted against the piston cam surface 226 of the first brake arm202. Extension of the piston 234 causes second brake aim 204 to rotatearound second axis 208, and causes the second brake pad holder 215 torotate in an inboard direction, eventually contacting an annular brakingsurface 126B of the wheel.

As this motion is occurring, the transfer cam surface 262 is lifting theroller 224 upward and leftward. This causes rotation of the first brakearm 202 around first axis 206. In the illustrated embodiment, the shapeof the transfer earn surface 262 ensures that the movement of brake padholders 210, 215 are “synchronous”, that is, that they move at the samerotational speed around axes 206, 208 but in opposite angulardirections. Since the brake pads 214, 218 are at the same radius fromtheir respective axes 206, 208, they will also move, in opposeddirections, at the same tangential speed.

FIGS. 10A-10B show force vectors operating on the various components ofthe brake. F_(b) is the brake force, or the pad force operating on theannular braking surfaces 126A, 126B to generate stopping power. F_(f) isa frame force, acting on the frame or fork mounts and orthogonal topivot axes 206, 208. F_(p) is the non-horizontal force of the piston234; an equal and opposite force between the piston 234 and the pistoncam surface 226. F_(r) is a roller force; an equal and opposite forcebetween the roller 224 and transfer cam surface 262. Φ is the anglerelative to the horizontal at which forces F_(p) and F_(r) act (see FIG.5). The ratio of F_(f)/F_(b) may be less than 2.5. Preferably, the ratioof F_(f)/F_(b) may be less than 2.0. More preferably, the ratio ofF_(f)/F_(b) may be less than 1.5. One technical advantage is that thepiston force F_(p) is spread among the roller force F_(r) and the frameforces F_(f) rather than just the frame forces F_(f) alone. This reducesloading on the brake mounts, creates a stiffer feel in actuating thebrake, and lowers frame/fork mount strength requirements.

An embodiment alternative to the one shown in FIGS. 2-14 is shown at1200 in FIGS. 15 and 16. In this embodiment, a first brake arm 1202 isrotatably mounted at axis 206 to a bicycle fork or frame member, and asecond brake arm 1204 is rotatably mounted to a fork or frame member ataxis 208, as before. But the centering adjust mechanism has been movedfrom second brake arm 1204 to first brake arm 1202. The first brake arm1202 has a downwardly depending brake pad holder 1206. An upwardlyextending biasing member 1208 is integral with brake pad holder 1206 androtates with holder 1206 around axis 206 as a unit. An upper end 1210 ofthe biasing member 1208 has a post 284 thereon that receives first end282 of the coil return spring 280. A force transfer member 1212 of thefirst brake arm 1202 is now split from the brake pad holder 1206 and mayrotate around axis 206 independently of it. The transfer member 1212continues to carry a contact surface such as cam roller 224 on itsremote end 1214.

A centering adjust set screw 1216 is threadedly received into a bore1218 in the biasing member 1208. A front end 1220 of the set screw 1216contacts a surface 1222 of the force transfer member 1212. Rotating theset screw clockwise or counterclockwise will change the angularseparation of the brake holder 1206 relative to the force transfermember 1212, thus permitting a centering adjustment of the brake 1200.

The second brake arm 1204 is no longer split. A brake pad holder 1224 ofthe second brake arm 1204 is integral with a force transfer member 1226thereof. The force transfer member 1226 contains the transfer camsurface 262 and the piston housing 230.

A further embodiment is shown in FIG. 17. This brake 1300 is similar tobrake 200 (FIGS. 2-14) but has no centering adjust mechanism at all. Thefirst brake arm 1302 and the second brake arm 1304 are not split.

The embodiment 1400 shown in FIG. 18 is similar to the one shown inFIGS. 15 and 16, but the connections of a return spring 1402 aredifferent. A first end 1404 of a helical return spring 1402 is connectedto an end 1210 of the first brake arm 1202. A second end 1406 of thereturn spring 1402 is affixed to a fastener 1408 that is coaxial withsecond axis 208. In this position, the return spring 1402 continues tourge brake pad holders 1410, 1412 apart and continues to urge spitmembers 1212 and 1410 together.

FIG. 19 illustrates another embodiment 1500 that generally is similar tobrake 1200. But in place of helical spring 1402 (FIG. 18), a torsionspring 1502 is used. A first end 1504 of the torsion spring 1502 isaffixed as by a fastener 1506 to biasing member end 1210. A second end1508 of torsion spring 1502 is affixed as by fastener 1408 to be coaxialwith the second axis 208.

FIG. 20 shows an embodiment 1600 in which the spring member 1602 isconstituted by a torsion spring that is wound around second axis 208. Afirst end 1604 of the torsion spring 1602 is positioned to abut aninterior surface 1605 of the brake holder 1606 of the second brake arm204. A second end 1608 of the torsion spring 1602 is joined to the firstbrake arm 202.

A cable pull embodiment 1700 is illustrated in FIG. 21. A first brakearm 1702 rotates as a unit around a first axis 206. A second brake arm1704 rotates as a unit around second axis 208. The first brake arm has afirst brake pad holder 1706 that downwardly depends from the axis 206. Aforce transfer member 1708 is formed to be integral with the pad holder1706 and extends inwardly, past center plane P, to an end 1710 that inthe illustrated embodiment is positioned outboard and upward from secondaxis 208. A contact surface, such as an arcuate roller cam surface 1712,is formed by the force transfer member 1708.

The second brake arm 1704 has a second brake pad holder 1714 thatdownwardly depends from second axis 208. A force transfer member 1716upwardly and inwardly extends from the second axis 208 and is integrallyformed with the second brake pad holder. In other embodiments, and inorder to provide a centering adjust mechanism as has been described inconjunction with other embodiments, brake pad holder 1706 may be splitfrom transfer member 1708, or brake pad holder 1714 may be split fromtransfer member 1716, and a set screw (not shown) or other centeringmember may be used to set the angular relationship between the splitmembers.

An end 1718 of the force transfer member 1716 has a contact surface suchas a roller 1720 which engages with the roller cam surface 1712 of theforce transfer member 1708. Transfer member 1716 further has an upwardlyextending lobe 1722 that is positioned to be on the same side of thecenter plane P as is end 1710 of the transferring member 1708. Aterminating fitting 1724 of a Bowden cable housing 1726 is pivotallyaffixed to the lobe 1722. A brake cable 1728 that is threaded throughthe housing 1726 of the Bowden cable has an end 1730 that is pivotallyattached to end 1710 by a suitable fastener 1732.

In the operation of this embodiment, the rider operates a hand leverlocated on the handlebar of the bicycle (not shown), creating tensileforce on the cable 1728. This will urge cable 1728 upward and intohousing 1726. Cable 1728 will draw transfer member end 1710 toward lobe1722. Tensile force on cable 1728 will also cause the arm 1702 to rotateas a unit around axis 206, inwardly pivoting first brake pad holder1706.

As this is happening, the roller cam surface 1712 lifts roller 1720 andtherefore the end 1718 of the transfer member 1716. This will cause thesecond brake arm 1704 to rotate, in an opposite direction, around axis208, pivoting second brake pad holder 1714 in an inboard direction.

The motion of brake pad holders 1706, 1714 may be synchronized bycorrectly choosing the shape of the roller cam surface 1712. In thisillustrated embodiment, the synchronizing roller cam surface 1712describes a circular arc around a center 1734 that is downward andinboard from the surface 1712.

The embodiments herein described have the technical advantage ofproducing linear actuation rates.

Numerous modifications to the embodiments disclosed herein will beapparent to those skilled in the art in view of the foregoingdescription. For example, any of the embodiments disclosed herein may bemodified to include any of the structures or/or methodologies disclosedin connection with different embodiments. Accordingly, this disclosureis to be construed as illustrative only and is presented for the purposeof enabling those skilled in the art to make and use the invention, andto teach the best mode of carrying out same. The exclusive rights to allmodifications which come within the scope of the appended claims arereserved.

We claim:
 1. A rim brake for a bicycle, the rim brake comprising: afirst brake arm having a first pivot adapted for pivotal attachment tothe bicycle, a first brake pad holder of the first brake arm downwardlydepending from the first pivot, the first brake pad holder configured tomove in a first arcuate direction around the first pivot; a second brakearm having a second pivot adapted for pivotal attachment to the bicycle,a second brake pad holder of the second brake arm downwardly dependingfrom the second pivot, the second brake pad holder configured to move ina second arcuate direction around the second pivot opposite the firstarcuate direction, the second pivot horizontally spaced from the firstpivot; and only one hydraulic piston assembly, the hydraulic pistonassembly including a piston slidably mounted in a piston housing of thesecond brake arm, the piston advancing along a piston axis responsive tohydraulic fluid pressure to cause a non-horizontal force to be exertedon the first brake arm to move the second brake pad holder in the secondarcuate direction around the second pivot, the piston axis extendingbetween the first and second pivots; and a force transfer mechanismbetween the first brake arm and the second brake arm that causes a forcetransfer from the first brake arm to the second brake arm, the forcetransfer mechanism configured to move the first pad holder in the firstarcuate direction about the first pivot as the piston advances, whereinthe force transfer mechanism includes a first force transfer member ofthe first brake arm, the first force transfer member upwardly extendingfrom the first pivot and a second force transfer member of the secondbrake arm, the second force transfer member upwardly extending from thesecond pivot, a first contact surface on the first force transfer memberin contact with a second contact surface on the second force transfermember to impart torque to the second force transfer member in thesecond arcuate direction around the second pivot.
 2. The rim brake ofclaim 1, wherein the second contact surface on the second force transfermember of the second brake arm is a transfer cam surface and the firstcontact surface on the first force transfer member of the first brakearm bears against the transfer cam surface.
 3. The rim brake of claim 2,wherein the transfer cam surface is so shaped that the first and secondbrake pad holders move at the same speed around the first and secondpivots.
 4. The rim brake of claim 2, wherein the transfer cam surface islocated closer to the second pivot than the piston.
 5. The rim brake ofclaim 2, wherein the transfer cam surface is located in an upper andinboard direction from the second pivot.
 6. The rim brake of claim 2,wherein the first brake arm includes a piston cam surface, an end of thepiston abuts the piston cam surface, the piston cam surface is locatedin an upper and inboard direction from the first pivot.
 7. The rim brakeof claim 6, wherein the piston cam surface and the transfer cam surfaceare so shaped that a transfer cam force acting on the transfer camsurface by the first force transfer member will always be parallel tothe piston axis.
 8. The rim brake of claim 1, wherein actuation of thepiston causes the first and second pad holders to move in opposedinboard directions at the same speed.
 9. The rim brake of claim 1,further including a spring which urges the first and second pad holdersin opposed outboard directions.
 10. The brake of claim 1, wherein thepiston is located in an upper direction from the second pivot.
 11. Therim brake of claim 1, wherein the piston axis pivots around the secondpivot upon actuation of the rim brake.
 12. The rim brake of claim 1,wherein the piston axis extends between the first and second pivots. 13.The rim brake of claim 2, wherein the first contact surface on the firstforce transfer member of the first brake arm is a roller, which rollsagainst the transfer cam surface.